Fix flicker

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed `RichLog.write` not respecting `width` parameter https://github.com/Textualize/textual/pull/4978
 - Fixed `RichLog` writing at wrong width when `write` occurs before width is known (e.g. in `compose` or `on_mount`) https://github.com/Textualize/textual/pull/4978
 - Fixed `RichLog.write` incorrectly shrinking width to `RichLog.min_width` when `shrink=True` (now shrinks to fit content area instead) https://github.com/Textualize/textual/pull/4978
+- Fixed flicker when setting `dark` reactive on startup https://github.com/Textualize/textual/pull/4989
 
 ## [0.79.1] - 2024-08-31
 

src/textual/app.py

@@ -1084,17 +1084,17 @@ def watch_dark(self, dark: bool) -> None:
         self.set_class(dark, "-dark-mode", update=False)
         self.set_class(not dark, "-light-mode", update=False)
         self._refresh_truecolor_filter(self.ansi_theme)
-        self.call_later(self.refresh_css)
+        self.call_next(self.refresh_css)
 
     def watch_ansi_theme_dark(self, theme: TerminalTheme) -> None:
         if self.dark:
             self._refresh_truecolor_filter(theme)
-            self.call_later(self.refresh_css)
+            self.call_next(self.refresh_css)
 
     def watch_ansi_theme_light(self, theme: TerminalTheme) -> None:
         if not self.dark:
             self._refresh_truecolor_filter(theme)
-            self.call_later(self.refresh_css)
+            self.call_next(self.refresh_css)
 
     @property
     def ansi_theme(self) -> TerminalTheme:
@@ -3130,7 +3130,10 @@ def refresh_css(self, animate: bool = True) -> None:
         stylesheet.set_variables(self.get_css_variables())
         stylesheet.reparse()
         stylesheet.update(self.app, animate=animate)
-        self.screen._refresh_layout(self.size)
+        try:
+            self.screen._refresh_layout(self.size)
+        except ScreenError:
+            pass
         # The other screens in the stack will need to know about some style
         # changes, as a final pass let's check in on every screen that isn't
         # the current one and update them too.